Question

I want to pass a parameter into a factory that will be used to set the attribute on an associated model. The associated model is created within the factory.

I have a Transaction model with a many-to-many link to itself through a join table TxLink.
I want to call link = FactoryGirl.create(:link_red_to_sub, sub: 10, red: 7) which will create two Transaction objects and a TxLink that links the two.

I get an error in the association line below because of the units: sub at the end. The error is “trait not defined”. I tried units: { sub } (with brackets) instead but no dice.

factory :tx_link do
    units "9.99"

    factory :link_red_to_sub do
      ignore do
        sub 0
        red 0
      end
      units { red }

      association :giver, factory: :transaction, units: sub
      association :taker, factory: :redemption, units: red
    end
  end

Answer 1

The association helper in factory_girl can’t take dynamic property arguments, but you can use a dynamic attribute to instantiate the association however you want:

factory :tx_link do
  units "9.99"

  factory :link_red_to_sub do
    transient do
      sub 0
      red 0
    end
    units { red }

    giver { create(:transaction, units: sub) }
    taker { create(:redemption, units: red) }
  end
end

The only downside of this is that the giver and taker associations will use the create strategy regardless of what strategy you’re building tx_link with, which essentially means that attributes_for and build_stubbed won’t work correctly for the tx_link factory.

The example I provided requires version 4.5.0.